Lodi, California - One morning, a cat named Mama was playing happily with her fellow feline, Sis, before the unthinkable happened.

As her owner Alyssa Redwine explained, Mama suddenly collapsed while playing, despite showing no previous signs of health issues.

She immediately rushed to the cat's aid, but tragically, it was too late.

"She seemed completely fine that morning, playful and affectionate like her usual self, and then she just collapsed at home," Alyssa said in an interview with Newsweek.

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"I held her as she passed."

The shock was profound, but the young woman wasn't the only one deeply affected – Sis the cat was too.

After her owner had placed Mama in a cardboard box and covered it up, Sis took a look straight away, and Alyssa documented her older cat's heartbreaking reaction to her deceased pal.

In the corresponding TikTok video, Sis first peers into the box before tentatively touching the blanket and climbing in. She sniffs Mama for a few seconds, then pats her head a few times before leaving the box again.

"Heartache is watching your cat say goodbye to her best friend for the last time," Alyssa wrote over the video. "Animals understand more than we think."

Sis had already been living with Alyssa for a year when the then-still-feral Mama joined in 2019, the 29-year-old said.

"The two of them formed an incredibly close bond very quickly," Alyssa said.

"They were inseparable, always curled up together, grooming each other, and following each other around."

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Now, humans and animals can at least comfort each other in the wake of Mama's passing.

But the California native admitted, "I think it's safe to say Sis and I will always feel like something is missing without our Mama Kitty around.

"She will be so greatly missed."