The boys were shocked all right, and they knew where this was going. She needed help braking into kranng headquarters, which is dangerous, and saving her family from the kranng, that's why she helped them. To show how helpful and strong she is, well played.
"Your family? What happened?" Mikey asks letting go of her arm. "It's a long story, but the truth is most of us were mutated as animals. They experimented on us and want to make us into weapons. That's why we made an escape plan, but I was the only one who got away. So I promised them I would brake them out when I find the help." They all look at each other then back to her "we will help you." Leo says with a smile, but then Raph cuts in "but, you have to help us fight the kranng." Leo gives him a "are you serious" look. "I'll do it," she says smiling and Leo smiles to "it's a deal!"
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At the lair....
Leo walks in with pizza boxes and small little figures for forming a plan. Bellatrix was by Donnie with his computer telling him where her family is, so he can get a map of the place. It didn't take long but soon Donnie got the details he needed. "Lucky you I still have this," she says pulling out one of the kranng's pass cards. Donnie looked like he was about to fall out of his chair, and he did. Lol.
"Grate, that will come in handy," Leo says walking by her. "We have a plan ready, now we just need your word, to make sure it works."
Bellatrix nods and walks over to hear the plan. Today, she will save her family.