What was The Doctor's advice? Has this ever happened before?
The doctor knows where Gallifrey is but the process of regeneration was going to destroy that information, so he's hidden it in the pocket watch then left himself clues. He's going to need that bloody pocket watch back... Idiot.
The doctor will scribble over everything with chalk until he falls into a bubble universe, destroy/trick/defeat/woo the gatekeeper (Missy) and find Gallifrey.
Missy will be someone we know. Her manner resembles River Song (who is presumably 'dead' after the events of the season final and thus able to appear in this weird visage of 'heaven' the robot called, 'the promised land'). It's possible the ring has appeared to remind him that he's married, he's almost certainly stored some of his memories in the pocket watch before regenerating (hence his failing memory) and the older incantation of the doctor is actually something River spent most of last season harping on about. Don't forget she's a Timelord and could both change form and be working with the other Timelords to help The Doctor rescue Gallifrey from the bubble universe.
If anything, 'IN THE FOREST OF THE NIGHT' sounds suspiciously like a call-back to 'FOREST OF THE DEAD'
If Missy is some version of River Song, then it fits neatly with the running theme of leaving people behind. He left her in a library computer to do then in a dying Tardis.
Missy could also be the Tardis... "He can be very mean you know, except to me because he loves me so much." There aren't many people that The Doctor truly loves - or can call him their 'boyfriend'. "I do like his knew accent though - think I might keep it." Is weird.
That makes it sound like Missy is part of the doctor herself. Re-watching her last scene - she speaks as though she were in the room when the Doctor was tormenting the robot guy. If she is part-doctor then his memory might also have ended up in her.
It's hard to tell from one episode but if Moffat abides by the law of mystery writing (which you'd hope he would considering his work on Sherlock) then we already have enough information to work out what's going on.