A 73-year-old Japanese woman presented with a mass in the right breast. Physical examination confirmed a mass in the lower inner quadrant of the right breast with a maximum diameter of about 3 cm, and the absence of axillary lymphadenopathy. Mammography showed a lobulated and high-density mass in the left lower inner quadrant of the breast (a). The margins of the mass were micro-serrated, there were no calcifications inside. Ultrasonography showed a well-circumscribed and amorphous mass measuring 2.7 cm in size. The mass had well-defined margins, enhanced posterior echoes, a halo, and low-level internal echoes (b). In ultrasonography, there was no observation of axillary lymphadenopathy. Contrast-enhanced magnetic resonance imaging (MRI) showed a 3.1 cm amorphous mass with a pattern of early enhancement and washout in the right breast (c). There was extensive ductal spread and a daughter nodule in the outside anterior of the mass. Core needle biopsy showed atypical spindle cells with cobblestone appearance (d), leading to diagnosis of IDC of no special type, T2N0M0 stage IIA. Immunohistochemistry confirmed that the lesion was positive for HER2 (score of 3+), negative for ER and PgR and 80% of MIB-1 positive cells. The patient received trastuzumab + pertuzumab + docetaxel (Tmab+Pmab+DTX) as HER2-targeted NAC. After three courses of Tmab+Pmab+DTX, ultrasonography showed the tumor growth to be 3.3 cm in size (a). The patient underwent two courses of epirubicin + cyclophosphamide (EC) as the next NAC regimen. However, the tumor was out of frame in ultrasonography (b) and grew to 3.8 cm in size with internal degeneration suspected of necrosis in MRI (c). She stopped receiving NAC and underwent a unilateral mastectomy and sentinel lymph node biopsy. The pathological tumor size was 3.5 × 2.5 cm (a), and the sentinel lymph node appeared to be normal. The postoperative pathological result of the surgical specimen showed that the tumor cells were pleomorphic and proliferated in sheets with necrosis and keratinization (b, c), leading to the diagnosis of metaplastic carcinoma (squamous cell carcinoma). Immunohistochemistry confirmed that the lesion was negative for ER and PgR and 80% of MIB-1 positive cells. The immunohistochemical score of HER2 was 2+, so we performed the fluorescence in situ hybridization test; the result was positive. The optimum postoperative treatment for MBC has not been determined, so we suggested follow-up observation could be an option. The patient nonetheless decided to receive adjuvant chemotherapy. She received trastuzumab emtansine as adjuvant chemotherapy, and there has been no recurrence as of 6 months after the surgery.