WATCH: Ginnifer Goodwin Breaks Down After Finding Out Her Family's Dark Secrets on Who Do You Think You Are?
07/24/2015 AT 05:10 PM EDT
"When my grandfather was alive he never wanted to discuss his parents," Goodwin, 37, tells PEOPLE. "A few years ago when I was still on Big Love, my father and I tried tracing my paternal great-grandparents on our own, but gave up pretty quickly. We didn't know what we were doing."
So recently Goodwin reached out to Who Do You Think You Are? for help. "They researched for about seven months before deciding that there was an abundant amount of material," Goodwin says. "It's astounding what they found."
On the season premiere, Goodwin discovers why her grandfather, who left home at age 11, kept his parents a secret for all of those years. "There is an incredible paper trail when your relatives were criminally inclined," Goodwin says.
In the exclusive clip above, Goodwin finds out that her paternal great-grandmother Nellie went to jail for dealing drugs, just one of several criminal activities her father's grandparents were involved in.
"I not only sobbed the whole week of filming, but my husband (Once costar Josh Dallas), watched it with me and it gutted us all over again," Goodwin says. "I never dreamt that I would be made to feel like I actually know these people. I got such an intimate look into their lives. I now understand that they were greatly misunderstood."
It's an episode that executive producer and The Comeback star Lisa Kudrow is extremely proud of. "A lot of people think their family history will be dull, but we know that if they were humans, they weren't dull," Kudrow tells PEOPLE. "It's unbelievable what we were able to find out for Ginnifer."
And the experience means the world to the Goodwin. "It's important for my son (Oliver, 13 months) to know his history," Goodwin says. "Life experiences become this patchwork, like a quilt that we wrap around us and our ancestors are very much a part of that. My great-grandparents didn't get the Goodwin love, so I feel like it's important for Oliver to somehow give them that love retroactively."
Who Do You Think You Are returns Sunday at 9 p.m ET on TLC.