Gas Mask is the mysterious person that followed the Liars in Ravenswood. This character first appeared at the end of "Now You See Me, Now You Don't". Showrunner Marlene King has stated that she believes "there were two gas masks that night" and further implying that the antagonist we saw was not Ezra.
The Liars discover a photo of a World War I solider's uniform and gas mask on a board in what they think may be A's lair, next to a flier for a World War I-themed celebration of Ravenswood later that night. Someone is seen in the costume at the end of the episode, getting ready for the party.
Aria gets a phone call while looking for Alison DiLaurentis with the girls. The person turns out to be Ezra, who is seen wearing the Gas Mask costume while spying on the girls. He is later seen following Hanna inside the old victorian mansion and locks her inside an antique phone booth in the house. Spencer later has an encounter with him in the greenhouse, and they begin to tussle. Spencer takes some gardening shears nearby and slashes his hand. Ezra successfully knocks her to the ground, but before Spencer is able to take off his mask, he wakes up and Spencer falls back and hits his head and gets knocked out.