|“||If I knew Nate was going to get Garrett out, I would have stayed in tonight. You have to get Maya's cellphone back. It shouldn't be too difficult. Paige doesn't even know she has it. Huh, it sucks we didn't get to make that phone call. Ring-ring! What's Paige doing with Maya's cellphone? O-M-G, she must be the killer. Oh, well, even the best laid plans go awry. [Pause] I'll see you soon.||”|
— Mona's monologue as Ali Dee to Toby
Ali Dee is the alter ego of Mona Vanderwaal. During Spencer's stay at Radley, she found out that Mona had hidden her nurse cap, CeCe Drake's visitor pass, and her own fake ID in a rocking horse. Spencer was led to the rocking horse by Mona's map (which depicted the rocking horse as a star) as well as instruction from Alison DiLaurentis (hallucination). It's also been speculated that Ali Dee is A.D., meaning that Mona could be A.D. using an alias.
It is unknown to the audience who "Ali Dee" is for sure. We do know that Mona Vanderwaal's face and identity are printed on an ID card to get out of Radley Sanitarium. She possibly used the name as a pun to Alison DiLaurentis's name. She also called Alison "Ali Dee" in that fake story she told Spencer.
Mona leaves Radley dressed as nurse. Later, she is seen wearing a black hoodie and taking instructions from someone by phone. She goes to "A's" lair and reports to someone, who also wears a black hoodie, that she changed plans. Near the end of the episode, Mona is talking to another member of The A-Team, who is revealed to be Toby. She tells him about her disappointment that the plans didn't work out as they should.
Spencer finds out about Mona's alter ego. She was led to the rocking horse in the children's quarters in Radley by a map made by Mona on a board game. Spencer also hallucinated Alison in the room who supposedly helped her find it.