。
She's caught on to my secret plan.
Literal
She [topic-は] my secret [of-の] plan [target-に] is-noticing.
気づく ('to notice, realize, become aware of') takes the noticed thing with に, not を — a quirk shared with several Japanese verbs of perception and awareness. The progressive ~ている turns the act of noticing into a sustained state of awareness — 'she has noticed (and the awareness persists).' 秘密の計画 stacks two Sino-Japanese compounds: 秘密 ('secret') + 計画 ('plan'), joined attributively with の.