Not sure what you mean. There were three parts to Indra's involvement in the Mahabharata.
First was his fathering of Arjuna. Next, because Karna (fathered by the sun Surya) was actually a better archer than Arjuna, Indra tricked Karna into giving up his golden breastplate that was a part of his body. This significantly weakened Karna. This was a fantastic scene, and certainly not one that showed Indra that well - Karna came off far more noble from this event. It is probably this event that you have heard about.
Lastly, Indra was of course present, as the lord of heaven, when Arjuna spent time there before the great war, but Indra played a fairly uncontentious role in that part.
We are reading it in the garden in then morning currently. We had a great section this morning with the amazing story of Amba and Bishma, where after such determination and suffering, Amba was reborn as Shikhandi who was destined to kill Bishma in the great war. It was an exhilarating tale. You can really see why Indians have loved this story of the Mahabharata for so long - it really is unsurpassed in its magic, wisdom, courage and spirituality.