diff --git a/none/command.py b/none/command.py
index 9d648168..c8994fde 100644
--- a/none/command.py
+++ b/none/command.py
@@ -1,10 +1,12 @@
 import re
+from collections import defaultdict
 from typing import Tuple, Union, Callable, Iterable, Dict, Any, Optional
 
 from aiocqhttp import CQHttp, Error as CQHttpError
 from aiocqhttp.message import Message
 
 from . import permissions as perm
+from .helpers import context_source
 
 # Key: str (one segment of command name)
 # Value: subtree or a leaf Command object
@@ -15,12 +17,8 @@ _registry = {}
 _aliases = {}
 
 # Key: context source
-# Value: Command object
-_sessions = {}
-
-
-# TODO: session 保存为一个栈，命令可以调用命令，进入新的 session，命令执行完毕，
-# 中间没有抛出异常（标志进入交互模式的异常），则从栈中 pop
+# Value: list (stack) of Session objects
+_sessions = defaultdict(list)
 
 
 class Command:
@@ -74,89 +72,6 @@ async def calculate_permission(bot: CQHttp, ctx: Dict[str, Any]) -> int:
     return permission
 
 
-def _find_command(name: Union[str, Tuple[str]]) -> Optional[Command]:
-    cmd_name = name if isinstance(name, tuple) else (name,)
-
-    if not cmd_name:
-        return None
-
-    cmd_tree = _registry
-    for part in cmd_name[:-1]:
-        if part not in cmd_tree:
-            return None
-        cmd_tree = cmd_tree[part]
-
-    return cmd_tree.get(cmd_name[-1])
-
-
-class Session:
-    __slots__ = ('cmd', 'ctx',
-                 'current_key', 'current_arg', 'current_arg_text',
-                 'images', 'args', 'last_interaction')
-
-    def __init__(self, cmd: Command, ctx: Dict[str, Any], *,
-                 current_arg: str = '', args: Dict[str, Any] = None):
-        self.cmd = cmd
-        self.ctx = ctx
-        self.current_key = None
-        self.current_arg = current_arg
-        self.current_arg_text = Message(current_arg).extract_plain_text()
-        self.images = [s.data['url'] for s in ctx['message']
-                       if s.type == 'image' and 'url' in s.data]
-        self.args = args or {}
-        self.last_interaction = None
-
-    def require_arg(self, key: str, prompt: str = '', *,
-                    interactive: bool = True):
-        # TODO: 检查 key 是否在 args 中，如果不在，抛出异常，保存 session，等待用户填充
-        pass
-
-
-async def handle_command(bot: CQHttp, ctx: Dict[str, Any]) -> bool:
-    # TODO: check if there is a session
-    msg_text = str(ctx['message']).lstrip()
-
-    for start in bot.config.COMMAND_START:
-        if isinstance(start, type(re.compile(''))):
-            m = start.search(msg_text)
-            if m:
-                full_command = msg_text[len(m.group(0)):].lstrip()
-                break
-        elif isinstance(start, str):
-            if msg_text.startswith(start):
-                full_command = msg_text[len(start):].lstrip()
-                break
-    else:
-        # it's not a command
-        return False
-
-    if not full_command:
-        # command is empty
-        return False
-
-    cmd_name_text, *cmd_remained = full_command.split(maxsplit=1)
-    cmd_name = _aliases.get(cmd_name_text)
-
-    if not cmd_name:
-        for sep in bot.config.COMMAND_SEP:
-            if isinstance(sep, type(re.compile(''))):
-                cmd_name = tuple(sep.split(cmd_name_text))
-                break
-            elif isinstance(sep, str):
-                cmd_name = tuple(cmd_name_text.split(sep))
-                break
-        else:
-            cmd_name = (cmd_name_text,)
-
-    cmd = _find_command(cmd_name)
-    if not cmd:
-        return False
-
-    session = Session(cmd, ctx, current_arg=''.join(cmd_remained))
-    # TODO: 插入 session
-    return await cmd.run(bot, session)
-
-
 def on_command(name: Union[str, Tuple[str]], aliases: Iterable = (),
                permission: int = perm.EVERYONE) -> Callable:
     def deco(func: Callable) -> Callable:
@@ -185,6 +100,159 @@ def on_command(name: Union[str, Tuple[str]], aliases: Iterable = (),
     return deco
 
 
+def _find_command(name: Union[str, Tuple[str]]) -> Optional[Command]:
+    cmd_name = name if isinstance(name, tuple) else (name,)
+
+    if not cmd_name:
+        return None
+
+    cmd_tree = _registry
+    for part in cmd_name[:-1]:
+        if part not in cmd_tree:
+            return None
+        cmd_tree = cmd_tree[part]
+
+    return cmd_tree.get(cmd_name[-1])
+
+
+class FurtherInteractionNeeded(Exception):
+    """
+    Raised by session.require_arg() indicating
+    that the command should enter interactive mode
+    to ask the user for some arguments.
+    """
+    pass
+
+
+class Session:
+    __slots__ = ('cmd', 'ctx',
+                 'current_key', 'current_prompt',
+                 'current_arg', 'current_arg_text',
+                 'images', 'args', 'last_interaction')
+
+    def __init__(self, cmd: Command, ctx: Dict[str, Any], *,
+                 current_arg: str = '', args: Dict[str, Any] = None):
+        self.cmd = cmd
+        self.ctx = ctx
+        self.current_key = None
+        self.current_prompt = None
+        self.current_arg = current_arg
+        self.current_arg_text = Message(current_arg).extract_plain_text()
+        self.images = [s.data['url'] for s in ctx['message']
+                       if s.type == 'image' and 'url' in s.data]
+        self.args = args or {}
+        self.last_interaction = None
+
+    def refresh(self, ctx: Dict[str, Any], *, current_arg: str = ''):
+        self.ctx = ctx
+        self.current_arg = current_arg
+        self.current_arg_text = Message(current_arg).extract_plain_text()
+        self.images = [s.data['url'] for s in ctx['message']
+                       if s.type == 'image' and 'url' in s.data]
+
+    @property
+    def is_valid(self):
+        # TODO: 检查 last_interaction
+        return True
+
+    def require_arg(self, key: str, prompt: str = None, *,
+                    interactive: bool = True) -> Any:
+        """
+        Get an argument with a given key.
+
+        If "interactive" is True, and the argument does not exist
+        in the current session, a FurtherInteractionNeeded exception
+        will be raised, and the caller of the command will know
+        it should keep the session for further interaction with the user.
+
+        If "interactive" is False, missed key will cause a result of None.
+
+        :param key: argument key
+        :param prompt: prompt to ask the user with
+        :param interactive: should enter interactive mode while key missing
+        :return: the argument value
+        :raise FurtherInteractionNeeded: further interaction is needed
+        """
+        value = self.args.get(key)
+        if value is not None or not interactive:
+            return value
+
+        self.current_key = key
+        self.current_prompt = prompt or f'请输入 {self.current_key}：'
+        raise FurtherInteractionNeeded
+
+
+def _new_command_session(bot: CQHttp,
+                         ctx: Dict[str, Any]) -> Optional[Session]:
+    msg_text = str(ctx['message']).lstrip()
+
+    for start in bot.config.COMMAND_START:
+        if isinstance(start, type(re.compile(''))):
+            m = start.search(msg_text)
+            if m:
+                full_command = msg_text[len(m.group(0)):].lstrip()
+                break
+        elif isinstance(start, str):
+            if msg_text.startswith(start):
+                full_command = msg_text[len(start):].lstrip()
+                break
+    else:
+        # it's not a command
+        return None
+
+    if not full_command:
+        # command is empty
+        return None
+
+    cmd_name_text, *cmd_remained = full_command.split(maxsplit=1)
+    cmd_name = _aliases.get(cmd_name_text)
+
+    if not cmd_name:
+        for sep in bot.config.COMMAND_SEP:
+            if isinstance(sep, type(re.compile(''))):
+                cmd_name = tuple(sep.split(cmd_name_text))
+                break
+            elif isinstance(sep, str):
+                cmd_name = tuple(cmd_name_text.split(sep))
+                break
+        else:
+            cmd_name = (cmd_name_text,)
+
+    cmd = _find_command(cmd_name)
+    if not cmd:
+        return None
+
+    return Session(cmd, ctx, current_arg=''.join(cmd_remained))
+
+
+async def handle_command(bot: CQHttp, ctx: Dict[str, Any]) -> bool:
+    src = context_source(ctx)
+    if _sessions[src]:
+        session = _sessions[src][-1]
+        session.refresh(ctx, current_arg=str(ctx['message']))
+        # TODO: 检查 is_valid
+    else:
+        session = _new_command_session(bot, ctx)
+        if not session:
+            return False
+        _sessions[src].append(session)
+
+    try:
+        res = await session.cmd.run(bot, session)
+        # the command is finished, pop the session
+        _sessions[src].pop()
+        if not _sessions[src]:
+            # session stack of the current user is empty
+            del _sessions[src]
+        return res
+    except FurtherInteractionNeeded:
+        # ask the user for more information
+        await bot.send(ctx, session.current_prompt)
+
+        # return True because this step of the session is successful
+        return True
+
+
 async def call_command(name: Union[str, Tuple[str]],
                        bot: CQHttp, ctx: Dict[str, Any], **kwargs) -> bool:
     """
diff --git a/none/helpers.py b/none/helpers.py
index ab668673..50b7b69c 100644
--- a/none/helpers.py
+++ b/none/helpers.py
@@ -6,11 +6,11 @@ from aiocqhttp import CQHttp, Error as CQHttpError
 def context_source(ctx: Dict[str, Any]) -> str:
     src = ''
     if ctx.get('group_id'):
-        src += 'g%s' % ctx['group_id']
+        src += f'/group/{ctx["group_id"]}'
     elif ctx.get('discuss_id'):
-        src += 'd%s' % ctx['discuss_id']
+        src += f'/discuss/{ctx["discuss_id"]}'
     if ctx.get('user_id'):
-        src += 'p%s' % ctx['user_id']
+        src += f'/user/{ctx["user_id"]}'
     return src
 
 
diff --git a/plugins/weather.py b/plugins/weather.py
index 366aceab..e3673904 100644
--- a/plugins/weather.py
+++ b/plugins/weather.py
@@ -6,7 +6,8 @@ from none.helpers import send
 @none.on_command('weather', aliases=('天气',))
 async def weather(bot, session: Session):
     city = session.require_arg('city', prompt='你想知道哪个城市的天气呢？')
-    await send(bot, session.ctx, f'你查询了{city}的天气')
+    other = session.require_arg('other')
+    await send(bot, session.ctx, f'你查询了{city}的天气，{other}')
 
 
 @weather.args_parser