。
She's in the middle of planning a new play.
Literal
She [topic-は] new play [object-を] planning-in-progress is.
The suffix ~中 (ちゅう, 'in the middle of, during') attaches to the verbal noun 企画 ('planning') to create 企画中 ('in the process of planning'). 芝居 (しばい, 'play, theater production') covers stage performances broadly. This compact construction — noun+中+だ — is a common way to describe an ongoing activity without using ~ている.