n medium humanoid 

Perception +7; darkvision

Skills Deception +11, Diplomacy +11, Society +8, Stealth +8

Str +3 Dex +3 Con +0 Int +1 Wis +2 Cha +4

AC  18; Fort +5, Ref +10, Will +11

HP 50

End the Charade [R] When the doppelganger is transformed with Change Shape and another creature moves adjacent to them or uses a Strike or another hostile action against them, the doppelganger can use its reaction to revert to its natural form and make a Strike against that creature. If the creature was unaware the doppelganger was in disguise, that creature has the flat-footed condition against this Strike.

Speed 25 feet

Melee [>] claw +10 (agile), Damage 2d6 + 5 slashing

Change Shape [>] (arcane, concentrate, polymorph, transmutation) The doppelganger takes on the specific appearance of any Small or Medium humanoid it has seen. This doesn't change the doppelganger's Stride speed or their attack and damage bonuses with their Strikes, but it might change the damage type their Strikes deal (typically to bludgeoning).

Mind Reading [>>] (arcane, manipulate) The doppelganger attempts to read the surface thoughts of one creature within 30 feet. The target resists if it succeeds at a DC 21 Will save.

Beginner Box, page 60.