Cancer is a condition where cells in a specific part of body grow and reproduce uncontrollably. The purpose of this study is to assess adverse events and change in disease activity when ABBV-400 is given in combination with Fluorouracil, Folinic Acid, and Bevacizumab to adult participants to treat unresctable metastatic colorectal cancer. ABBV-400 is an investigational drug being developed for the treatment of unresectable metastatic colorectal cancer. Fluorouracil, folinic acid, and bevacizumab (FFB) is an approved drug for the treatment of unresectable metastatic colorectal cancer. Study doctors put the participants in groups called treatment arms. Each treatment arm receives a different dose of ABBV-400 in combination with FFB in escalating doses on two different schedules (safety lead in), followed by low or high doses of ABBV-400 in combination with FFB or fluorouracil, folinic acid, irinotecan, and bevacizumab (standard of care \[SOC\]) \[dose optimization\] on its own, ending with low or high doses of ABBV-400 in combination with FFB for continued dose optimization and expansion. Approximately 280 adult participants with unresectable metastatic colorectal cancer will be enrolled in the study in 65 sites worldwide. In the safety lead in, participants will receive escalating intravenous (IV) ABBV-400 in combination with IV FFB on two different schedules. During the dose optimization participants will receive IV ABBV-400 in combination with IV FFB at low or high doses determined in the safety lead in. The dose optimization arm will also include a comparator cohort in which participants will receive SOC. During the dose optimization and expansion stage, participants will receive IV ABBV-400 in combination with IV FFB at low or high doses that have been determined from the previous stages. The study will run for a duration of approximately 3 years. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at an approved institution (hospital or clinic). The effect of the treatment will be frequently checked by medical assessments, blood tests, questionnaires and side effects.