Tonight’s episode is called “The Donation Oscillation.” If you remember, Leonard has offered to donate his sperm to impregnate the wife of Penny’s ex-boyfriend. As that plan comes closer to fruition, so to speak, Penny becomes unexpectedly jealous – and decides to seduce Leonard just before he leaves for his mission of mercy, to weaken the potency of his donation. That’s all the network episode description says, so I don’t know how Penny plans to entice him – but if she dresses up as Princess Leia, it’ll be borrowing a page from Rachel’s playbook on Friends.