"소방차를 찾아야 한다!"
Translation:You have to find the fire engine!
~아/어야 하다 (or 되다) = have to / must [verb]. In this case, it's 하다 which is conjugated in the plain form.
Because 하다 is the dictionary form and you can't end a sentence in the dictionary form. The plain form conjugation, however, is valid. (Other conjugations are, of course, 해요, 합니다, 해, etc)