First, the question: If a player doesn't remember being subjected to a Geas, is the Geas still in effect?
Second, the setup for context:
I am a DM in a game where one player left for several weeks and unexpectedly re-joined the game. The plot device I used to re-join the character to the party is that an NPC that the party had also previously traveled with showed up with the PC. There was little exposition as to what exactly happened - when and where the NPC found the PC.
Unbeknownst to the players, the NPC is a traitor and they are walking into an ambush.
What I would like to do is set up a situation where the PC is actually under the affect of a Geas spell to help the NPC in combat against the rest of the party when a 'trigger word' is spoken (and still within the standard 30 day duration of Geas).
In order to prevent the PC from remembering that they are under this compulsion, I was going to argue they've been subjected to Modify Memory to avoid that. It may be considered unfair, but in order to further the plot and raise the stakes, I'm not bothering with saving throws. My argument is that the PC must have failed the saves for these spells. Otherwise, they wouldn't be here right now (the NPC would have just killed them) and would have remembered those events. They don't remember, ergo they must have failed them.